Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 24 Page 25 Page 26 Page 27 Page 28 Page 29 Page 30 Page 31 Page 32 Page 33 Page 34 Page 35 Page 36 Page 37 Page 38 Page 39 Page 40 Page 41 Page 42 Page 43 Page 4428 IREC Farmers' Newsletter No. 197 — Autumn 2017 windmill grass samples collected mainly from non-crop areas on cotton farms,” he said. “Preliminary results from the samples collected across cotton farming systems in 2016–17 show 95% of fleabane samples, 60% of feathertop Rhodes grass samples, 80–90% of windmill grass samples and 20% of sowthistle samples tested as resistant to glyphosate.” “Keep in mind that flaxleaf fleabane, feathertop Rhodes grass and windmill grass are not registered for control with Roundup Ready® Herbicide with PLANTSHIELD (used with Roundup Ready® technology crops). Control of these species is generally poor because of naturally tolerant plants within these species, so there is further selection for resistance which only exacerbates the problem,” he said. “Although still lower compared with other weeds, the level of resistance to glyphosate emerging in sowthistle is very concerning, given that this species is listed on the herbicide label. According to a recent attitudinal study by Monsanto, growers also identify annual ryegrass and barnyard grass as showing signs of glyphosate resistance in the field.” To date there have been no recorded cases of paraquat resistance in weeds on cotton farms, however with rising glyphosate resistance, and increased use of paraquat products, there is a high risk that paraquat resistance will also be found, leaving growers with few options to control these weeds. Mr Koetz now has additional funding available to test the seed collected in the survey for resistance to paraquat and diquat products — Gramoxone® and Spray.Seed®. Mr Koetz said the lack of diversity in herbicide use in many cotton systems is likely to contribute to the increased incidence of herbicide resistance. “The attitudinal study by Monsanto indicates that less than 50% of growers are applying a pre-emergent herbicide and only 25% of growers apply a post-emergent herbicide in addition to their applications of glyphosate in cotton,” he said. “The label for Roundup Ready® Herbicide with PLANTSHIELD states that this product must not be the only form of weed control used in Roundup Ready Flex cotton varieties.” “In the next five to 10 years there will need to be a shift towards non-herbicide controls such as robotic cultivation and microwave technologies, which are well suited to summer cropping on rows or beds,” he said. “Until then, optical spray technology is a good option for growers to keep weed numbers low in the fallow.” The timing of fallow sprays is presenting itself as a critical issue on cotton production areas. Spray drift of Group I herbicides (e.g. 2,4-D products) applied late in the summer fallow to control large fleabane is causing considerable damage to neighbouring cotton crops. Mr Koetz recommends growers change their fallow weed management program to target small plants earlier in the spring, before cotton emergence, using a double knock of glyphosate followed with cultivation or paraquat plus a residual herbicide as the second knock. Protecting knockdown herbicide options Most cropping systems in the irrigation regions (and dryland areas) rely fairly heavily on a small group of non-selective or ‘knockdown’ herbicides. Since the widespread adoption of zero and minimum tillage, these herbicides have provided effective control of many grass and broadleaf weeds but these useful herbicides could be lost to the industry if steps are not taken to increase the diversity of weed management tactics used. Mark Congreve, senior consultant with Independent Consultants Australia Network (ICAN), said that the highly effective double knock tactic, which combines an application of glyphosate followed by paraquat, is at risk if growers don’t remain vigilant and ensure removal of any surviving plants. “The double knock strategy of glyphosate, plus a Group I herbicide for weeds such as flaxleaf fleabane, followed by paraquat has provided excellent control of weeds that are difficult to kill with glyphosate alone,” he said. “Recent confirmation of a fleabane population that is resistant to paraquat, found in a New South Wales vineyard, is a clear warning to Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knockdown herbicides in irrigated farming systems. Mark Congreve, ICAN senior consultant, says growers need to be looking for survivor weeds after every herbicide application and responding to ‘rate creep’ by changing how they use herbicides across their cropping system.